这又是一个文件比较命令，熟练掌握会节省你很多工作。还记得comm命令吗？diff比他更加复杂，好用，不仅能比较文件，还能比较目录。 当一个文件有多个版本时候，或者更多复杂的文件，目录比较时，你会期望有一个比comm更有用的命令，diff正是为此而生。
全称：different file
实例：
文件1： v1.txt
hello
world v1_echo
v1_diff
v1_comm
shell is easy!
文件2： v2.txt
hello world!
v2_echo
v1_diff
easy shell!
1.普通比较：
$diff v1.txt v2.txt
1,2c1,2
< hello world
< v1_echo
---
> hello world!
> v2_echo
4c4
< v1_comm
---
> easy shell!
列出比较的区别，数字为行数，c是compare
2.一体化输出：
$diff -u v1.txt v2.txt > version.patch
-v1_echo
+hello world!
+v2_echo
v1_diff
-v1_comm
+easy shell!
shell is easy!
"+"表示补充的行，"-"表示删去的行
3. 更新覆盖v1.txt,注意这里借用了patch
$patch –p1 v1.txt< version.patch
4. 比较v1,v2文件，仅输出v2的变化
$diff -e v1.txt v2.txt
4c
easy shell!
.
1,2c
hello world!
v2_echo
.
5.比较目录
借用书上一段解释：
-n 将所有确实的文件视为空文件
-a 将所有文件市委文件文本
-u 生成一体化输出
-r 遍历目录下的所有文件
比较两个目录下所有文本信息
$diff –naur directory1 directory2
除非你确定这两个目录基本相同，不然不推荐这么用。
6.工作疲劳后，尤其同时做很多事情的时候，会忘了有没有做过上传，或者修改某个目录下的文件。可以这样：
$diff v1.txt /${dir}
直接比较同名文件，高强度下的工作者相信真有体会。
解读下—help
太长，截取少部分解读一下。
mandatory arguments to long options are mandatory for short options too.
--normal output a normal diff (the default)
-q, --brief report only when files differ
-s, --report-identical-files report when two files are the same
-c, -c num, --context[=num] output num (default 3) lines of copied context
-u, -u num, --unified[=num] output num (default 3) lines of unified context
-e, --ed output an ed script
-n, --rcs output an rcs format diff
-y, --side-by-side output in two columns
files 的格式可以是‘file1 file2'、‘dir1 dir2'、‘dir file...'或
‘file... dir'。
解读：
1.-q –s
-q: 只有两个不同是，提示“两个文件不同”
-s: 两个文件相同时，提示“两个文件相同”
$diff -qs v1.txt v2.txt
(有点无聊的命令 )
2. –y
按照列的方式输出，增加了直观性
hello world | hello world!
v1_echo | v2_echo
v1_diff v1_diff
v1_comm | easy shell!
shell is easy! shell is easy!
3. file 后面的格式
可以是1.文件 文件 2.目录与目录 3.文件与目录 4. 目录与文件
解释一下，文件与目录的形式，比较的是该目录下 与 比较文件同名的文件比较
如：$ diff v1.txt diff/
则实际是v1.txt 与 diff/v1.txt的比较。这是个很常用的功能。
对比comm
就比较文件而言，comm功能比较简单，但是也很直观，方便
而diff 提供了更强大，更复杂的比较方式，还能对比目录，遍历目录。
用法：diff [选项]... files
compare files line by line.
mandatory arguments to long options are mandatory for short options too.
--normal output a normal diff (the default)
-q, --brief report only when files differ
-s, --report-identical-files report when two files are the same
-c, -c num, --context[=num] output num (default 3) lines of copied context
-u, -u num, --unified[=num] output num (default 3) lines of unified context
-e, --ed output an ed script
-n, --rcs output an rcs format diff
-y, --side-by-side output in two columns
-w, --width=num output at most num (default 130) print columns
--left-column output only the left column of common lines
--suppress-common-lines do not output common lines
-p, --show-c-function show which c function each change is in
-f, --show-function-line=re show the most recent line matching re
--label label use label instead of file name
(can be repeated)
-t, --expand-tabs expand tabs to spaces in output
-t, --initial-tab make tabs line up by prepending a tab
--tabsize=num tab stops every num (default 8) print columns
--suppress-blank-empty suppress space or tab before empty output lines
-l, --paginate pass output through `pr' to paginate it
-r, --recursive recursively compare any subdirectories found
-n, --new-file treat absent files as empty
--unidirectional-new-file treat absent first files as empty
--ignore-file-name-case ignore case when comparing file names
--no-ignore-file-name-case consider case when comparing file names
-x, --exclude=pat exclude files that match pat
-x, --exclude-from=file exclude files that match any pattern in file
-s, --starting-file=file start with file when comparing directories
--from-file=file1 compare file1 to all operands;
file1 can be a directory
--to-file=file2 compare all operands to file2;
file2 can be a directory
-i, --ignore-case ignore case differences in file contents
-e, --ignore-tab-expansion ignore changes due to tab expansion
-z, --ignore-trailing-space ignore white space at line end
-b, --ignore-space-change ignore changes in the amount of white space
-w, --ignore-all-space ignore all white space
-b, --ignore-blank-lines ignore changes whose lines are all blank
-i, --ignore-matching-lines=re ignore changes whose lines all match re
-a, --text treat all files as text
--strip-trailing-cr strip trailing carriage return on input
-d, --ifdef=name output merged file with `#ifdef name' diffs
--gtype-group-format=gfmt format gtype input groups with gfmt
--line-format=lfmt format all input lines with lfmt
--ltype-line-format=lfmt format ltype input lines with lfmt
these format options provide fine-grained control over the output
of diff, generalizing -d/--ifdef.
ltype 可以是‘old'、‘new'或‘unchanged'。gtype 可以是 ltype 的选择
或是‘changed'。
gfmt (only) may contain:
%< lines from file1
%> lines from file2
%= lines common to file1 and file2
%[-][width][.[prec]]{doxx}letter printf-style spec for letter
letters are as follows for new group, lower case for old group:
f first line number
l last line number
n number of lines = l-f+1
e f-1
m l+1
%(a=b?t:e) if a equals b then t else e
lfmt (only) may contain:
%l contents of line
%l contents of line, excluding any trailing newline
%[-][width][.[prec]]{doxx}n printf-style spec for input line number
both gfmt and lfmt may contain:
%% %
%c'c' the single character c
%c'\ooo' the character with octal code ooo
c the character c (other characters represent themselves)
-d, --minimal try hard to find a smaller set of changes
--horizon-lines=num keep num lines of the common prefix and suffix
--speed-large-files assume large files and many scattered small changes
--help display this help and exit
-v, --version output version information and exit
files 的格式可以是‘file1 file2'、‘dir1 dir2'、‘dir file...'或
‘file... dir'。
if --from-file or --to-file is given, there are no restrictions on file(s).
如果 file 是‘-'，则由标准输入读取内容。
如果输入相同，则退出状态为 0；1 表示输入不同；2 表示有错误产生。
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